Communication traffic is increased day by day, but the communication band is limited. For a limited band communication system, the capacity or the spectral efficiency is an important issue. Obviously, the spectral efficiency of present communication system, such as GSM (Globe System for Mobile Communication) and narrowband CDMA, is not satisfied.
The third generation (3G) mobile communication system being researched will greatly increase capacity and spectral efficiency, by using of some new technologies, such as smart antenna, multi-user detection and spread-spectrum etc. The 3G mobile communication system includes the IMT 2000 family, such as Wideband Code Division Multiple Access (WCDMA), Time Division—Synchronous Code Division Multiple Access (TD-SCDMA), CDMA 2000 etc.
The smart antenna uses techniques, such as spatial filtering and beam forming etc. Since space division concept is used, a subscriber is differentiated with other subscribers not only by “code”, but also by “physical space”. In this way, the multiple access interference (MAI) between different subscribers is greatly reduced. It is obvious that the smart antenna has advantage for increasing system performance and spectral efficiency and will have a bright application future. It is definite that the smart antenna will be used in the TD-SCDMA system. Besides, the smart antenna technique is considered to use in the 3G mobile communication system too.
Multiuser detection technique, including joint detection and interference suppression etc, is used in reception. Since the MAI is suppressed, so capacity of a system will be increased. At present, the technique has been used in many communication systems.
Core of the spread-spectrum technology is that data of every subscriber are modulated by spreading code with pseudo-random characteristics. For example, in a CDMA system subscribers are differentiated by different spread codes. The spreading code can use various pseudo-random sequences. Some communication systems even use combined spread code, i.e., combining two or more than two kinds of spread codes. For example, in IS95 and 3G families, a WALSH code including OVSF code and a scrambling code including Gold code, m sequences are combined as a spread code. OVSF code is an orthogonal variable spreading factor codes, its generation method can refer to the Chinese Wireless Telecommunication Standard (CWTS) TS C104 V3.0.0 Spreading and Modulation. In a 3G communication system, for a certain length of OVSF code, the number of spreading codes equals to the length of the spread code. For example, when the spreading code length is 16, the number of the spreading codes is also 16.
In general, a spread-spectrum communication system is a system with interference limited. The capacity of a spread-spectrum communication system is determined by the interference level within the system. In a spread-spectrum communication system, there are various kinds of interference, such as additive white Gaussian noise (AWGN), multipath interference and multiple access interference caused by multiple subscribers, etc. For a specific mobile communication system, when the number of subscribers is increased (capacity is increased), multiple access interference becomes the key to limit the increasing. This is because the multiple access interference is stronger along with the increasing of subscribers number and power.
In summary, the capacity of a spread-spectrum communication system is limited by interference of the system, the used bandwidth and the number of spread codes, etc. When the system uses smart antenna technique, the multiple access interference between subscribers will be reduced. When the system also uses multiuser detection at the same time, the multiple access interference will be further reduced, and the capacity and performance of system will be raised.
In general, a spread-spectrum communication system without using smart antenna and multiuser detection techniques cannot use all number of channels provided by the spread codes, i.e., the system real capacity cannot reach the design capacity. Usually, a spread-spectrum communication system that only uses smart antenna technique or multiuser detection technique alone cannot suppress multiple access interference perfectly. For example, a SCDMA system with an OVSF code of length 16 only uses zero-forcing joint detection algorithm, without using smart antenna technique, then the system performance is rapidly worse when system noise is increased. Therefore, in order to increase capacity of a spread-spectrum communication system, it is necessary to use smart antenna and multiuser detection techniques at the same time.
The TD-SCDMA system proposed by CWTS uses smart antenna and multiuser detection techniques at the same time, so the system capacity is greatly increased and the system performances are much better.
Nevertheless, although the present spread-spectrum communication system uses both techniques: smart antenna and multiuser detection, but the number of channels of the spreading codes set is not increased so that the advantage of smart antenna and multiuser detection can not be brought into play. This is another limitation of a system capacity. For example, in a TD-SCDMA system using OVSF codes, when the length of spreading code is 16, the simultaneous channels number (subscribers number) cannot excess 16.